1. Field of the Invention
The present invention relates to a gallium nitride (GaN) group semiconductor laser device incorporated as a light source in an optical system of an optical pickup apparatus for an optical disk and the like.
2. Description of the Related Art
GaN group (GaInAlN) semiconductors are used as materials for semiconductor laser devices having an emission wavelength in a range of ultraviolet to green light wavelengths. A semiconductor laser device using GaN group semiconductors is described in MRS Internet J. Nitride Semiconductor Res. Vol. 2 (1997) Art. 5, for example. FIG. 4 shows a cross-sectional view of this conventional semiconductor laser device. Referring to FIG. 4, the semiconductor laser device includes a sapphire substrate 201, a GaN buffer layer 202, an n-type GaN contact layer 203, an n-type In0.05Ga0.95N layer 204, an n-type Al0.08Ga0.92N cladding layer 205, an n-type GaN guide layer 206, an active layer 207 of a multi-quantum well (MQW) structure composed of In0.15Ga0.85N quantum well layers and In0.02Ga0.98N barrier layers, a p-type Al0.2Ga0.8N layer 208, a p-type GaN guide layer 209, a p-type Al0.08Ga0.92N cladding layer 210, a p-type GaN contact layer 211, a p-side electrode 212, and an n-side electrode 213. Specifically, the MQW structure active layer 207 is composed of a total of seven layers, i.e., four In0.15Ga0.85N quantum well layers each having a thickness of 3.5 nm and three In0.02Ga0.98N barrier layers each having a thickness of 7 nm, which are alternately stacked. In this conventional semiconductor laser device, the p-type Al0.08Ga0.92N cladding layer 210 and the p-type GaN contact layer 211 are etched to form a stripe-shaped ridge for narrowing current to be injected.
When a GaN group semiconductor laser device is used as a light source of an optical disk system, such a laser device is of a self-oscillation type which outputs modulated light power for injection of a constant current, so as to prevent an occurrence of data read error due to noise generated during data read. A semiconductor laser device of this type is described in Japanese Laid-Open Publication No. 9-191160. FIG. 5 shows a cross-sectional view of this conventional semiconductor laser device. Referring to FIG. 5, the semiconductor laser device includes an n-type SiC substrate 221, an n-type AlN buffer layer 222, an n-type AlGaN cladding layer 223, an n-type GaN optical guide layer 224, an In0.05Ga0.95N quantum well active layer 225 having a thickness of 10 nm, a p-type GaN optical guide layer 226, a p-type AlGaN cladding layer 227, a p-type In0.1Ga0.9N saturable absorption layer 228 having a thickness of 5 nm, a p-type GaN contact layer 209, a p-side electrode 230, and an n-side electrode 231. In this conventional semiconductor laser device, part of light generated in the active layer 225 is absorbed by the saturable absorption layer 228. This changes the absorption coefficient of the saturable absorption layer 228, and with this change of the absorption coefficient, the intensity of laser-oscillated light emitted from the active layer 225 cyclically changes.
As a result, the interference of output light from the semiconductor laser device decreases. If a semiconductor laser device having a low interference is used as a light source of an optical disk system, output light from the semiconductor laser device does not interfere with return light which has directly returned to the active region of the semiconductor laser device after being reflected from a disk. This suppresses generation of noise and thus prevents occurrence of data read error.
When the semiconductor laser device with the above construction is incorporated as a light source in an optical system of an optical pickup apparatus for an optical disk and the like, a tracking servo mechanism is required to accurately focus a spot of a laser beam emitted from the semiconductor layer device on a pit array formed on a surface of the disk. This tracking servo mechanism normally employs a technique called a three-beam method for detecting a displacement of a spot from a pit.
FIG. 6 schematically shows an optical pickup apparatus employing the above technique. Referring to FIG. 6, laser light 242 emitted from a semiconductor layer device 241 is split into three beams by a diffraction grating 243. The split beams pass through a non-polarizing beam splitter 244 and a collimator lens 245 to be collimated. The collimated beams are then focused by an object lens 246 on an information recording surface of a disk 247 on which a pit array is formed. The three beams focused and reflected from the information recording surface of the disk 247 are guided back to the non-polarizing beam splitter 244 via the object lens 246 and the collimator lens 245, to be received respectively by photodiodes 248, 249, and 250. The photodiode 248 functions to read a signal representing a pit array recorded on the information recording surface of the disk 247, while the photodiodes 249 and 250 function to detect a displacement of a spot of a laser beam from a pit. The positions of the object lens 246 and the like are adjusted in accordance with the outputs from the photodiodes 249 and 250, so that a spot of a laser beam can be accurately focused on a pit array formed on the surface of the disk.
In the above three-beam method, three beams reflected from the information recording surface of the disk 247 are not only reflected from the non-polarizing beam splitter 244 to be received by the photodiodes 248, 249, and 250, but partly pass through the non-polarizing beam splitter 244 to be incident on the diffraction grating 243. The incident converged beam is divided into three beams by the diffraction grating 243 to illuminate the surface of the semiconductor laser device 241 as return light. In FIG. 6, the illumination positions of the three return beams are denoted by A, B, and C.
FIG. 7 is a front view of the semiconductor laser device 241 for illustrating the illumination positions of the three return beams. At the illumination position A, the return beam directly returns to the active region of the semiconductor laser device 241. The illumination positions B and C of the return beams are away from the illumination position A downward and upward, respectively, by a distance of about 20 xcexcm to 50 xcexcm. In the conventional semiconductor laser device shown in FIG. 5, the saturable absorption layer 228 is provided for suppressing generation of noise due to interference between output light of the laser device and return light at the illumination position A.
The conventional GaN group semiconductor laser device has the following problems.
Three return beams are produced in the case of employing the three-beam method shown in FIGS. 6 and 7. Among the three beams, the return beam at the illumination position B is incident on the substrate of the semiconductor laser chip. If the substrate is made of a material having a small absorption coefficient with respect to laser light, such as sapphire and silicon carbide, the return beam at the illumination position B is subjected to multiple reflection inside the substrate forming an interference pattern. The conventional GaN group semiconductor laser device uses sapphire or silicon carbide as a material of the substrate. Further, no layer for absorbing laser light is formed between the active layer and the substrate. It has been found, therefore, that an interference pattern formed by the return beam at the illumination position B and the laser light in the active region interact with each other, resulting in influencing the intensity of the output light of the semiconductor laser device.
A disk is rotated in an optical disk system so that data is read from the disk. During the rotation, the disk tends to tilt slightly, and the angle of this tilt varies with the rotation of the disk. This variation in the tilt angle of the disk causes the illumination position B to change slightly and thus change the interference pattern formed by the return beam at the illumination position B. As a result, the intensity of the output light of the semiconductor laser device is influenced and varied. If the intensity of the output light varies, such a semiconductor laser device is not practically usable as a light source of an optical disk system.
As described above, the conventional GaN group semiconductor laser devices are not prepared for being incorporated in an optical system of an optical pickup apparatus employing the three-beam method.
The gallium nitride group semiconductor laser device of this invention includes an active layer made of a nitride semiconductor formed between cladding layers and/or guide layers made of a nitride semiconductor on a substrate, wherein a light absorption layer is formed between the substrate and one of the cladding layers located closer to the substrate, the light absorption layer being made of a semiconductor having an energy gap substantially equal to or smaller than an energy gap of the active layer.
In one embodiment of the invention, the light absorption layer is of a multi-quantum well structure including two types of semiconductors having different compositions which are alternately stacked.
In another embodiment of the invention, the light absorption layer is made of a nitride semiconductor containing at least indium and gallium.
In still another embodiment of the invention, the substrate is made of a material selected from the group consisting of sapphire, gallium nitride, and silicon carbide.
In still another embodiment of the invention, the thickness of the light absorption layer is 0.05 xcexcm or more.
According to another aspect of the invention, an optical pickup apparatus is provided. The optical pickup apparatus is of a three-beam method including at least a semiconductor laser device and a diffraction grating, wherein the semiconductor laser device is a gallium nitride group semiconductor laser device according to the present invention.
The gallium nitride (GaN) group semiconductor laser device according to the present invention includes an active layer made of a nitride semiconductor sandwiched between cladding layers and/or guide layers made of nitride semiconductors formed on a substrate. A light absorption layer is formed between the substrate and one of the cladding layers located closer to the substrate, i.e., the lower cladding layer. The light absorption layer is made of a semiconductor having an energy gap substantially equal to or smaller than that of the active layer. With this construction, return light which is incident on the substrate after being reflected from a disk is blocked from entering the active layer by the light absorption layer formed between the substrate and the lower cladding layer. Accordingly, an interference pattern, which may be generated in the substrate by the return light, is prevented from interacting with laser light in the active region, keeping the intensity of output light of the semiconductor laser device from being influenced by the interference pattern. The resultant GaN group semiconductor laser is free from data read error and thus can be practically used as a light source of an optical disk system.
Since the absorption layer is provided between the substrate and the lower cladding layer, laser light propagating in the active layer of the semiconductor laser device does not expand to reach the absorption layer. The absorption layer therefore does not absorb laser light propagating in the semiconductor laser device, but absorbs only return light incident on the substrate from the disk. Thus, the laser characteristics such as the oscillation threshold current value and the maximum light power are not degraded.
Further, if a nitride semiconductor containing at least indium and gallium is used as a material of the light absorption layer, the formation of the light absorption layer is facilitated since the energy gap of this layer can be reduced only by increasing the content of indium. Moreover, since the light absorption layer is made of the same group of nitride semiconductor material as that used for the active layer and the cladding layers, the crystallinity will not be lost during the formation of the multi-layer structure by crystal growth. This is markedly advantageous for improving the reliability of the laser device.
It has been further found that the reliability improves if the light absorption layer is formed of a nitride semiconductor containing at least indium and gallium with a larger content of indium between the substrate and the lower cladding layer, compared with the case where no light absorption layer is formed. Normally, a GaN group semiconductor laser device has a difference in thermal expansion coefficient among a substrate and respective layers formed thereon. Thermal distortion is therefore generated while the temperature is being lowered to room temperature after crystal growth. Such thermal distortion generates a stress, which in return facilitates expansion of a defect existing in the crystal, causing degradation of the semiconductor laser device. According to the present invention where the light absorption layer made of a nitride semiconductor containing a large content of indium is formed, the volume elasticity is small due to the large content of indium. Such a light absorption layer can relieve the thermal distortion and thus relieve and reduce the stress in the crystal. As a result, expansion of a defect in the crystal is not facilitated, and thus the reliability of the semiconductor laser device improves.
Such a light absorption layer is not necessarily a single layer, but may be of a multi-quantum well structure composed of two types of semiconductors having different compositions alternately stacked. If a multi-quantum well structure is adopted, the densities of state at the ends of the conduction band and valence band increase due to a quantum effect, increasing light absorption. In addition, since interfaces between stacked semiconductor layers reflect light, return light incident on the substrate from a disk can be absorbed with high efficiency.
The lattice constant of the light absorption layer made of a nitride semiconductor containing at least indium and gallium described above becomes large as the content of indium thereof increases. Accordingly, a distortion may be generated if the thickness of this layer is large, resulting in a loss of crystallinity. This problem is overcome by adopting the multi-quantum well structure composed of two types of nitride semiconductors having different indium contents alternately stacked. That is, one of the nitride semiconductors having a smaller content of indium can relieve this distortion. Thus, the light absorption layer can be formed without the loss of crystallinity.
According to the present invention, the light absorption layer provided between the substrate and the lower cladding layer prevents return light incident on the substrate from a disk from entering the active layer. In consideration of this point, the present invention is especially effective for a GaN group semiconductor laser device having a substrate made of sapphire, gallium nitride, or silicon nitride which has a small light absorption coefficient with respect to laser light output from the active layer made of a nitride semiconductor.
Furthermore, in order to sufficiently absorb return light incident on the substrate from a disk, the thickness of the light absorption layer is preferably 0.05 xcexcm or more. FIG. 3 shows the measurement results of the attenuation of the intensity of laser light having a wavelength of 410 nm with respect to the thickness of an In0.2Ga0.08N light absorption layer. As is observed from this figure, when the thickness of the light absorption layer is 0.05 xcexcm or more, the light intensity is sufficiently attenuated, indicating that return light incident on the substrate from a disk is prevented from entering the active layer. The same results are obtained for a light absorption layer made of other material of GaN group semiconductor and for a light absorption layer of a multi-quantum well structure composed of two types of semiconductors having different compositions alternately stacked. Thus, the use of a light absorption layer having a thickness of 0.05 xcexcm or more enables realization of a GaN group semiconductor laser device practically usable as a light source of an optical disk system without causing data read error.
Thus, the invention described herein makes possible the advantages of (1) providing a gallium nitride group semiconductor laser device capable of being used as a light source of an optical disk system, and (2) providing an optical pickup apparatus using such a gallium nitride group semiconductor laser device.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.